Polar vapor sensing means

ABSTRACT

The sensing means comprises a body of metal oxide dielectric material having an active surface layer exhibiting surface conductive characteristics wherein the resistivity of the surface layer varies in the presence of a polar vapor and wherein the resistivity of the surface layer varies between a first condition in the absence of water vapor and a second condition in the presence of water vapor by a factor on the order of 1: 10,000 or more. At least two separate spaced electrically conductive electrodes are electrically connected with portions of the surface layer. Means is provided for impressing an AC voltage across said electrodes, and a load impedance is connected in series therewith from which an output signal can be derived.

United States Patent [72] Inventors Charles F. Pulvari Washington, D.C.;Stephen F. Urban, Kenmore, N.Y. [21] Appl. No. 5,354 [22] Filed Jan. 23,1970 [45] Patented Nov. 16, 1971 [73] Assignee NL Industries, Inc.

New York, N.Y. by said Urban Continuation-impart of application Ser. No.709,642, Mar. 1, 1968, now abandoned.

[54] POLAR VAPOR SENSING MEANS a 18 Claims, 7 Drawing Figs. [52] US.317/231, 317/262, 324/61 [51] Int. Cl 01g 9/00 [50] Field 01 Search317/230, 231, 238, 258, 262; 106/39; 324/61 [56] Reierences Cited UNITEDSTATES PATENTS 2,924,814 2/1960 Simpson 317/262 Primary Examiner-lamesD. Kallam Attorneys-Charles F, Kaegebehn, Robert L. Lehman and FredFloersheimer ABSTRACT: The sensing means comprises a body of metal oxidedielectric material having an active surface layer exhibiting surfaceconductive characteristics wherein the resistivity of the surface layervaries in the presence of a polar vapor and wherein the resistivity ofthe surface layer varies between a first condition in the absence ofwater vapor and a second condition in the presence of water vapor by afactor on the order of 1: 10,000 or more. At least two separate spacedelectrically conductive electrodes are electrically connected withportions of the surface layer. Means is provided for impressing an ACvoltage across said electrodes, and a load impedance is connected inseries therewith from which an output signal can be derived.

POLAR VAPOR SENSING MEANS CROSS REFERENCE TO THE RELATED APPLICATION Thepresent application is a continuation-in-part of copending US. Pat.application Ser. No. 709,642, filed Mar. 1, 1968, now abandoned.

BACKGROUND TO THE INVENTION The apparatus according to the presentinvention may be used in any application wherein it is desired to detect.the presence of certain polar vapors. It may advantageously be employedfor example to detect the humidity of ambient air and may be connectedin a suitable electric circuit for indicating the amount of humidity orfor operating humidity control mechanisms. It may also be employed inmany other fields as for detecting the polar vapors of crude oils duringoil prospecting operations. Additionally, many other applications of theinvention willsuggest themselves to one skilled in the art. For example,the invention may be used in the medical, biological, manufacturing andcontrolling fields as well as in chemical processes and the like.

Prior art devices for measuring humidity have employed many differentarrangements. Typically, such devices have utilized hairs, strings andthe like for operating a suitable linkage. Electrical circuits have alsobeen employed wherein a resister in the circuit may be provided with anorganic coating.

Such prior art structures for measuring humidity have not proved to besatisfactory since the components thereof were subject to breakage orother physical damage, and further due to the fact that the resultsobtained therewith were not sufficiently reliable due to the nature ofthe substances utilized as the sensing means thereof.

SUMMARY OF THE INVENTION The present invention provides a so-calledsolid state sensing means in the form of a body of metal oxidedielectric material including an active surface layer having surfaceconductive characteristics and in a dry condition at room temperaturehaving a resistance on the order of IO -l ohms between its electrodes.Certain additive means may be incorporated in the body to enhancechanges in the electrical surface properties of the body. At least twoseparate spaced electrically conductive electrodes are electricallyconnected with spaced portions of said surface layer.

This type of sensing means does not comprise organic substances, is veryrugged and not as susceptable to physical damage as prior artarrangements. Furthermore, the sensing means of the present invention isvery reliable, the sensing means when removed from a polar vaporatmosphere retuming to its original sensitivity condition or dryresistance whereby the repeatability of the apparatus is very good. Itshould be noted that due to the fact that the operation of this sensingmeans is based on the active surface properties of the metal oxide bodyits response is fast and can be expressed in seconds rather than inminutes, which is a most important feature when it is used forautomatically controlling the conditions of large rooms.

The term polarizable metal oxide dielectric material" includes a groupof materials which may be termed Moxies, this expression includingferroelectric crystals, ferrielectric crystals, ferroelectric glassceramics including ferroelectrics grown in a glassy matrix, fused quartzand various other glassy compositions such as silica glasses. All ofthese materials are capable of being treated so as to provide a thinactive surface layer having surface conductive characteristics ashereinafter described. These materials are also insoluble in water.

The term ferroelectric crystal" as used herein is intended to denote acrystal which has ferroelectric properties such as a Curie point andwherein the polarization thereof can be reversed with an electric fieldlower than the breakdown voltage of the crystal. A typical example ofsuch a crystal is also intended to include ferrielectric crystals suchas bismuth titanate which have characteristics similar to ferroelectriccrystals and additionally include a threshold switching field when thevoltage applied thereto is reversed.

In the sensing means of the present invention, the capacity andresistivity between the electrodes of the device vary in the presence ofpolar vapors and this variation is essentially a surface effect occuringdue to the interaction of the dipoles adsorbed on the surface ratherthan absorbed in the bulk of the crystal. It has been found that ahighly polished crystal surface was more sensitive and gave betterrepeatable results than a frosted surface which indicates that onlyadsorption of the polar vapors and no absorption is required to obtainthe sensing effect.

Although the exact mechanism of the surface conductivity induced by thepolar vapors is not yet known, it is believed that the surfaceconductivity is a result of the fact that the surface layer includesatomic arrangements such as hydroxyl groups which are active withrespective to polar molecules so as to produce rearrangement of theatoms of the polar molecules when in contact therewith thereby producingcharge carriers. The continuous free carrier or charge productionoccuring on the active surface acts like a donor for the semi-insulatingsurface layer to thereby substantially decrease the dry resistance ofthe sensing means in the presence of polar vapors.

The active surface layer exhibits a surface conductivity when subjectedto polar vapors. This surface conductivity is believed to be a result ofmolecular interaction at the activated metal oxide surface whichrearranges the molecular structure of the polar vapor, and as a result,active groups are created on the surface of the metal oxide, Forexample, in the case of water vapors and a silicon-based glass, theactive groups formed are surface hydroxyl groups spaced sufficiently farapart so that they do not interact with one another, surface hydroxylgroups which are so close together that they are hydrogen-bonded to oneanother, and molecular water which is physically adsorbed on the surfaceof the glass. In this case, the first two groups are of major interestfrom the viewpoint of adsorption.

The nature of the active surface is controlled by the temperature atwhich the metal oxide dielectric material is melted and the loss ofvolatile components during such melting, the loss of volatiles from thesurface during the activating process and the impurities and vaporsintroduced are adsorbed during the forming and cooling processes.

Although each of the various processing phases have an effeet on theformation of active groups on the surface of the metal oxide dielectricmaterial, the thermal history of the activating process is mostsignificant. It should be noted that some impurities enhance theformation of active surface groups while others inhibit the formation ofsuch active surface groups. Various types of metal oxide dielectricmaterials may form different active surface groups whereby the surfaceis responsive to various polar vapors. It has been found that in case ofwater vapor adsorption which has been extensively studied, therehydration of silica or bismuth titanate metal oxide surfaces dependson the previous thermal history. Up to temperatures of about 400 C., thedehydroxylation of the surface is reversible, but at temperatures above400 C., the removal of the adjacent hydroxyl groups from the surfacecauses the surface to become hydrophobic. It was found that a heattreatment or activation step was effectively obtained when the surfaceof the metal oxide dielectric material was subjected to a temperature ofabout 500 C. for an extended period of about 1 to 2 hours.

It has been found that due to the interaction of a polar vapor and theactive surface layer of a Moxi, the capacity of a capacitor defined bythe Moxi as a dielectric and its associated electrodes changes dependingupon how much polar vapor is deposited on the free crystal surface.

In addition, the resistance between the two electrodes changes as aconsequence of the amount of deposited polar vapor. Although the exactphysics of this phenomenon is not entirely clear, it is apparent thatthe factors responsible in obtaining a high sensitivity sensing means isa thin active skin layer and the interaction of the field of polarmolecules with the active skin layer. An external electric field isapplied to the electrodes through a load impedance from which an outputsignal can be derived.

The sensing ability of the active surface layer may be due to threefactors, namely, the active surface properties of the sensing means, theinteraction of polar vapors with said active surface, and some ratherweak semiconductive properties of the surface layer of the Moxi whichhas in a dry condition resistance comparable to insulators and thereforeis more correctly called a semi-insulator.

The metal oxide dielectric materials used in the present invention areessentially insulators with a resistance between electrodes spaced about12-20 mils apart on the order of l-l0 ohms in a dry condition. If a thinsurface layer is activated as described hereinabove, the interaction ofpolar vapors and the active surface layer causes a resistance todecrease to a range on the order of -10 ohms. This represents a changein resistance of the surface layer between a first condition in theabsence of water vapor and a second condition in the presence of watervapor by a factor on the order of 1:10.

The bulk of the body of metal oxide dielectric material does not changeits resistance and is not effected by the polar vapors. The activesurface layer is so thin that it can be removed by scratching thesurface with an abrasive paper in which case the sensitivity of thesensing means vanishes.

The term semi-insulators" is used for insulating materials which due tosurface conditions as described above or impurities render an insulatingmaterial such as ceramic, glass or Moxies to become slightly conductiveand normally the lower range of resistance on the order of l0l0 ohms isobtained when subjected to polar vapors as contrasted to a resistance ofl0-l0 ohms between electrodes when not subjected to polar vapors.

The utilization of such high resistivity changes was not possible untilthe recent development of high impedance input integrated operationalamplifiers, and their small size made it feasible to transform the highimpedance changes of the semiinsulating surface conductive layer of themetal oxide dielectric material into low impedance and high currentoutputs.

Although it has been found that the surface layer of a number of Moxieshave been successfully converted to a metal oxide active layer whichresponds to the presence of polar molecules such as humidity in the formof water vapor and the like in that the capacity and/or resistivitybetween the electrodes of the sensing means changes in the presence ofsuch vapors when disposed on said active surface layer, it has beenfound that bismuth titanate exerts a particularly strong effect,probably because this material possesses a low dielectric constant.Bismuth titanate possesses a dielectric constant of e' l9 and a veryhigh Curie temperature such as approximately 675 C. Furthermore, the Tiatom lends itself very well to form very active surface groups.

The existence of a threshold switching field in bismuth titanate permitsthe maintenance of a relatively large driving voltage without causingswitching of the domains. This is one reason that the response ofbismuth titanate crystals may be superior to other ferroelectrics. Othercrystals possessing a threshold switching field and exhibiting goodresponse are sodium potassium niobate, sodium niobate vanadate, lithiumniobate and barium sodium niobate.

It has been found that the presence of certain additive means in theMoxi body and the active surface layer formed thereon in some instancessubstantially enhances the changes in the electrical surface conductiveproperties of the sensing means when subjected to polar vapors. Certainadditives have been found to be especially effective when sensingparticular polar vapors as hereinafter fully described. Crystals grownwith particular additive means as hereinafter discussed exhibitextraordinarily strong changes in the dielectric and conductivityproperties when subjected to polar vapors.

The presence of such additive means on the crystal surface apparentlyacts like catalyzers act in chemical processes since it has been clearlyascertained that the presence of particular additive means renders thecrystal responsive to particular chemicals present in polar vapors. Thecapacity and resistance changes of the sensing means may be measured ina suitable electrical circuit as hereinafter described. Means isprovided for impressing an AC voltage across the electrodes and a loadimpedance is connected in series with said means for impressing the ACvoltage and the active surface layer.

It has been found that the sensing means has maximum sensitivity orresponse when a ferroelectric crystal is employed and when the ACdriving voltage applied to the electrodes of the sensing means issmaller but close to the coercive voltage of the device kept on a valuewhich does not switch the domain configuration of the crystal. Typicaloperation frequencies are in the range of 20-500 kilocycles, but higherfrequencies also provide a good response.

The sensing means must also be so constructed that the free crystal areaof that part of the crystal upon which electrodes are mounted must belarge compared to the electrode area. If the electrodes coversubstantially the entire crystal area, the response of the sensing meansis practically nil. It has been found that the free crystal area must beat least as much as the C ,/C ratio as hereinafter defined, andgenerally, the free crystal area should be at least ten times largerthan the area of the electrode. Sample devices which do not incorporatethe proper ratio between free crystal area and electrode area have notprovided sufficient measuring range. Additionally, it has been foundthat the electrode should have a large contour or peripheral dimensionfor its area in order to be most effective.

The objectives of the present invention are to provide a new and novelpolar vapor sensing means which can be adapted for measuring orcontrolling humidity or for detecting certain polar vapors such ashydrogen sulfite and the like which are present in crude oils andwherein the apparatus could be employed for detecting the presence of acrude oil source and would pennit test holes to be drilled to a lessdepth for ascertaining the presence of oil in a particular drillingarea. The apparatus of the presence invention is quite simple andinexpensive in construction, and yet at the same time, it is quitecompact and versatile in application. The sensing means is additionallyapplicable to many different fields such as chemical process control.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a front view of a sensingmeans according to the present invention illustrating schematicallythereon the presence of domain walls within the crystal havingoppositely polarized areas at either side thereof;

FIG. 2 is a top view of a modified form of the invention;

FIG. 3 is a top view of still another form of the invention;

FIG. 4 is a longitudinal section through still another form of theinvention;

FIG. 5 is a schematic wiring diagram illustrating an electrical circuitincluding the sensing means of the present invention;

FIG. 6 is a schematic wiring diagram of still another electrical circuitemploying the present invention; and,

FIG. 7 is a schematic wiring diagram of yet another electrical circuitincorporating the sensing means according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the illustrated embodiments,ferroelectric crystals are described. It should, of course, beunderstood, that in each case any suitable body of metal oxidedielectric material may be employed as discussed hereinbefore, and thatthe ferroelectric crystals as disclosed are merely for the purpose ofillustration.

Referring now to the drawings, a first form of the invention isillustrated in FIG. 1 wherein a ferroelectric crystal is indicated byreference numeral 10. This crystal comprises a single crystal platewhich may comprise any of the conventional or aforementionedferroelectric and ferrielectric crystals. in the preferred embodiment,the crystal is formed of bismuth titanate having a relatively high Curietemperature as aforedescribed. Electrode means is provided in the formof a pair of electrically conductive electrodes 12 and 14 which aredisposed in contact and suitably secured to the opposite faces of thecrystal. These electrodes may for example comprise a conductive oxidedeposition formed in the usual manner. In a typical example the crystalmay have a thickness of approximately 2 mils with the length and widththereof being approximately two-eighths of an inch and one-eighth of aninch, while the electrodes may have a diameter of approximatelyone-sixteenth of an inch, wherein the electrodes are deposited in asubstantially circular configuration. Suitable electrical leads 16 and18 are secured to electrodes 12 and 14 respectively for connecting thesensing means in an associated electrical circuit.

The crystal is indicated as having a multiplicity of domain wallsschematically indicated by the lines 20, the surface of the crystalincluding a plurality of oppositely polarized areas as indicated by thepositive and negative signs adjacent the surfaces thereof, adjacentoppositely polarized areas being separated by one of the domain walls.When the polar vapors are deposited on the surface of the crystal, aspointed out previously, the capacity of the capacitor defined by member10, 12 and 14 changes in accordance with the amount of polar vapordeposited on the crystal surface.

it appears that the sensing device in effect constitutes a devicesimilar to a mass spectrograph responsive to polar vapors in general. itshould be noted, however, that the sensing means of the presentinvention is very much simpler than any other device which serves asimilar function. it is clear that the change in capacity andresistivity between the electrodes of the device is more a surfaceeffect occuring essentially on the free crystal area not covered by anelectrode rather than in the bulk of the crystal since the time responseof 40 the sensing means to a particular polar vapor occurs rather fast,in seconds or minutes, depending on the hysteresis cycles.

Bismuth titanate, among others, exhibited a very large response to polarvapors and particularly to humidity if hafnium, chromium, niobium andtantalum additives are included in small amounts within the range ofabout 0.05 to 3 mole percent to provide optimum results. The amount ofadditive should be within the extreme ranges of approximately 0.001 to10 mole percent.

7 An AC voltage is impressed through leads 16 and 18 on electrodes 12and 14, and optimum operating conditions were obtained when the additivemeans comprised tantalum, and optimum operation was also obtained whenthe additives comprised hafnium, chromium or niobium.

it has also been found that the firing temperature of electrodes 12 and14 provides best results if carried out at approximately 400 C. since inthe case of water vapors the dehydroxylation of the surface isreversible.

in the method of making the sensing means of the present invention, thesurface portions of the body of metal oxide dielectric material to whichthe electrodes are to be connected are preferably initially highlypolished to provide a mirror polished, smooth surface.

The electrodes are then suitably fixed to the surface of the body as byfiring the electrodes on as previously described, the temperature offiring being lower than the heat treatment temperature where the bodysurface is activated as hereinafter described. The electrodes may alsobe connected to the body by a vacuum deposition process or by a hightemperature hydrolysis process.

The active surface layer having surface conductive characteristics maybe formed on the Moxi by several different processes. This activesurface layer may firstly be formed by a heat treatment process whereinthe Moxi body is heated to a temperature of at least about 500 C. for atleast about i hour.

The thin active surface layer may also be formed by irradiating thesurface by electrons or ions such as performed in a conventional ionimplantation process or a conventional deoxidation process as willappear to one skilled in the art.

The Moxi body and the electrodes connected thereto are mounted in asupport socket and a protective means is then mounted in place relativeto the socket.

The relationship of the free crystal area to the electrode area shouldbe within the ranges as aforedescribed, and the free crystal area inthis case represents on the faces of the crystal on which the electrodesare disposed the total area not covered by the electrodes. The voltageimpressed on the electrodes should be within the ranges aforedescribed,and especially should be smaller but near to the coercive voltage of thecrystal if the material has ferroelectric properties.

As mentioned previously, certain additive means are included in thecrystal to enable the desired results. Reference is now made to table Iwhich-represents results obtained when various additives are included ina bismuth titanate crystal.

The first column of the table labeled GROUP represents the differentelements which comprise the additive means in a bismuth titanatecrystal, these elements having been given their conventional chemicalsymbol.

The next column represents capacitance and resistance ratios obtainedwhen the polar vapor comprises water. The next column represents theresults obtained when the polar vapor was linseed oil, and thesubsequent columns indicate the ratios obtained when the polar vaporswere oleic acid, dimethyl formarnide and acetic acid respectively.

An explanation is now in order as to what the ratios indicated in thesevertical columns represent. The first ratio defined is C,/C. C,

TABLE 1 Dimothyl formarnldo H O ratios Linseed oil ratios Oleic acidratios ratios Acetic acid ratios Group C. C./O GJG C./C G./G 0.]0 (L/UC./C (14G C./C 0.](1

Sr 23. 5 3 1 1 1 1 14 7. (l 36 5. ti Ba 1.3 8.2 1 1 1 1 1.3 1.0 2.3 16.0

1 1 1 1.4 2.4 2.8 iii represents the capacitance of the capacitorillustrated in FIG. 1 when the polar vapors deposited thereon weredeposited from ambient air at 100 percent humidity. C represents thecapacitance of the capacitor illustrated in FIG. 1 when the ambient airfrom which the polar vapor is deposited is substantially free ofmoisture. It will be noted that the results vary considerably inaccordance with the particular elements which form the additive means inthe crystal, and further dependent upon what particular polar vapor isto be detected.

The second ratio indicated in this table is G,/G. G, represents theresistivity of the capacitor shown in FIG. 1 when the polar vapor is at100 percent humidity and G represents the resistivity when the polarvapor is substantially free of moisture. In each case involving bothratios, it should be understood that an AC voltage is impressed acrossthe elecroom temperature and in air at room temperature. The additivesincluded in the crystal were within the range of approximately 0.001 to10 mole percent. From an inspection of table I, it is apparent thatpolar vapors have a profound effect on the capacitive and conductiveproperties of ferroelectric or ferrielectric capacitors. Furthermore, itshows that certain additives enhance the capacity variation responsesuch as strontium, lanthanum, zirconium and chromium. Some otheradditives enhance the resistivity variation response such as lanthanum,niobium, tantalum and molybdenum for example. In some cases, the twoeffects are about equal. In some cases, the resistivity change dominatesthe response. In other cases, the capacity change effect dominates theresponse.

Table I illustrates how this effect can be utilized for measurement andcontrol of humidity, for example, and wherein certain bismuth titanatecrystals having additives such as zirconium, hafnium, niobium andtantalum are especially well suited. In selecting the additives, thestrength of the response and also the time of response must beconsidered, the time of response usually being about 5 minutes, whilefor certain stronger responding additives such as chromium, the responsetime was as high as minutes, which is relatively slow.

It also may be seen from table I that other polar vapors may giveexcellent response with certain additives in bismuth titanate. Forexample, manganese provides a good response in a bismuth titanatecrystal when it is desired to detect acetic acid, while the sameadditive gives a rather small response for humidity.

In another example, the additive Samarium, while showing a very loweffect for humidity, shows a very high resistivity variation fordimethyl formamide.

It has also been found that the rate earths may be included as additivesin the ferroelectric crystals when it is desired to detect hydrogensulfite which is present in the polar vapor of crude oils.

Referring now to FIG. 2, modification of the invention is illustratedwherein a crystal 30 is provided, this crystal being formed of anysuitable ferroelectric or ferrielectric substance as describedhereinabove. The domain walls are again indicated schematically byreference numerals 32, and the posi tive negative signs indicate theoppositely polarized area separated by the domain walls. The few domainsillustrated schematically in this figure are only examples of domains asviewed from the top, these domains usually being distributed over theentire crystalline structure in a sort of semirandom fashion.

Similar electrodes may be suitably secured to opposite sides of thecrystal, one of the electrodes 34 being visible in FIG. 2. Thiselectrode is indicated generally by reference numeral 34 and includes agenerally cylindrical central portion having a plurality of elongatedportions 38 radiating outwardly therefrom. A suitable electric lead 40is connected with the electrode, and it will be understood that asimilar electrode structure is provided on the opposite side of thecrystal.

This particular electrode configuration provides a large contour orperipheral dimension for the area of the electrode, which has been foundto be more effective than a simple circular electrode as discussed inconnection with FIG. 1.

Various other electrode configurations may be employed in order toincrease the peripheral dimension for a given area of electrode. It isagain emphasized that the free crystal area should be large relative tothe electrode area and the ratios of the free crystal area to theelectrode area should be at least 10 times as great as the electrodearea, and in some cases, as

many as 50 times or more as great.

Referring now to FIG. 3 of the drawings, a crystal 50 similar to thecrystals previously described is provided, and in this form of theinvention, the two electrodes in contact with the crystal are bothdisposed on one face of the crystal. The first electrode includes acentral portion 52 having a plurality of elongated portions 54 extendingradially outwardly therefrom. An slsst l ad i sslsq tht s ss bs otherelectrode includes a generally circular portion 58 having elongatedportions 60 extending radially inwardly therefrom and in spacedrelationship to the elongated portions 54 of the other electrode. Anelectrical lead 64 is connected with the second electrode.

It is apparent that this arrangement insures a maximum contour orperipheral dimension for each of the spaced separate electrodes for thearea involved.

Here again, the free crystal area should be substantially greater thanthe area of the electrodes on the same order as previously described. Inthis instance, the free crystal area comprises that portion of thesurface of the crystal which is visible in FIG. 3 and is not covered bythe electrodes. The free crystal area in this modification would notinclude the opposite face of the crystal, since in defining free crystalarea, only that surface of the crystal upon which the electrodes aredisposed is included.

Referring now to FIG. 4 of the drawings, a single crystal plate 70similar to those previously described is provided, an electrode 72 beingsecured to one surface thereof and defining a plurality of elongatedportions 74 extending therefrom. A similar electrode 76 is afiixed tothe opposite face of the crystal and is provided with the sameconfiguration to increase the contour or peripheral dimension for thearea of the electrode.

A header 80 is provided, and two lead connections 82 and 84 aresupported within a body of insulating material 86 carried by the header.This insulating material may be similar to that employed in transistorheaders. Lead connections 82 and 84 are secured to electrodes 72 and 76respectively by conductive connecting portions 90 and 92 respectivelysuch as silver paste or by welding and the like. Here again, therelative sizes of the free crystal area and the electrode area conformwith the aforementioned requirements.

The crystal of the sensing means is protected by a cap 96 which slipsover the header and has a large number of holes 98 formed therethroughwhich permit a polar vapor to reach the surface of the crystal. It isapparent that many other types of protective housings may be providedfor supporting and protecting the sensing means of the presentinvention.

Referring now to FIG. 5, an electrical circuit is illustratedincorporating the sensing means of the present invention. The capacitorsensing means of any of the previously described fonns of the inventionmay be employed as the capacitor 100 which is one member of a balancedwheatstone bridge. The bridge balance can be set by the variablecapacitor 102 and the variable resistor 104 and by proper choice ofresistor 108. An CA voltage is impressed across terminals and 112 whichcauses the bridge is be driven across terminals 110 and 118, thepotential difference between terminals 114 and 116 is zero for thebalanced bridge.

The terminals 114, 116 and 118 of the bridge are connected to acomparator 120 which may comprise a differential amplifier. The outputof the amplifier is in turn connected with an electrical indicatinginstrument 122 such as a milliammeter or the like.

When the sensing capacitor 100 is subjected to polar vapors, thecapacity or resistivity or both varies and upsets the balance of thebridge. As a result, the output of comparator 120 supplies a voltage orcurrent to the indicating device and the amount of vapor present in theenvironment can be read out. It is apparent that a suitable recordinginstrument could also be employed in place of the indicating means 122.

Referring now to FIG. of the drawings, the sensing capacitor 130 may beof the construction illustrated in any of the forms shown in FIGS. 1-4,for example, in accordance with the present invention. This capacitor isalso connected in a wheatstone bridge arrangement including a variablecapacitor 132, a variable resistor 134 and a resistor 136. The means forproviding the AC drive in this form of the apparatus comprises aconventional small transistor Colpit oscillator 150 connected through acoupling capacitor 152 with the wheatstone bridge. The oscillator isprovided with a suitable DC current in Ee usu al manner. The oscillatorprovides the required alt ernating current and proper voltage which isset so that it does not switch the ferroelectric crystal sensing elementaround the 3 hysteresis loop.

The bridge balance can be set by the variable capacitor 132 and thevariable resistor 134 and by proper choice of resistor 136.

The output of the wheatstone bridge is connected with the inputtenninals 154 and 156 of an operational amplifier 160. The amplifier isa commercially available Amelco type integrated circuitry operationalamplifier, type 809C, and manufactured by Amelco Semiconductor, Divisionof Teledyne, Inc., 1300 Terra Bella Avenue, Mountain View, California.This is a well known and widely used operational amplifier prepared on avery small silicon chip encased in a T05 can.

Between the output terminal 162 and the input terminal 154 of theamplifier, a negative feedback is provided through resistor 164.Betweenthe terminals 166 and 168 of the amplifier, resistor 172 andcapacitor 174 are connected to provide compensating components to avoidoscillations of the amplifier. The resistor 176 connected between inputterminal 156- and ground also forms part of the compensating network. Apositive battery voltage is connected to tenninal 180, and the negativebattery voltage is connected to terminal 182.

When the operational amplifier is fed from a balanced bridge, no outputappears at terminal 162. However, if the sensing means or capacitor 130of the present invention is subjected to polar vapor, the capacity orresistivity or both change according to the amount of polar vaporpresent and the bridge is unbalanced. Accordingly, the output of theoperational amplifier will be proportion to the amount of vaporsdeposited on the surface of the crystal of the sensing means 130.

The output terminal 162 of the amplifier is connected through resistors190 and 192 to a siliconcontrolled rectifier 194 which has a loadimpedance 196 series with a positive voltage source. Thesilicon-controlled rectifier is biased by a negative potential source198. As the potential on the output terminal 162 of the amplifierincreases, it is possible to set by varying the resistor 190 a certainlevel at which the siliconcontrolled rectifier becomes conductive andswitches a suitable control device such as a relay, motor and the likeby placing the driving coil or resistance of such a control device inplace of the load impedance 196.

Accordingly, it is possible to set a level for initiating control of ahumidifier or dehumidifier so that suitable humidity control apparatusmay be energized and deenergized by the circuit. By setting the biaslevel, the switching operation may be controlled so as to produce adesired level of humidity. It is apparent that instead of thesilicon-controlled rectifier, a mag netic relay or transistor switch andthe like may also be employed.

The use of an operational amplifier in the circuit illustrated in FIG. 6increases the sensitivity of the arrangement considerably and alsoprovides a linear relationship between the ferroelectric crystal sensingmeans output and the output of the operational amplifier.

Referring now to FIG. 7 of the drawings, two electronic oscillators Z QQan d 202 are provided, these oscillators being disposed within a sealedenvelope indicated schematically by dotted line 204.

Oscillator 200 is used as a reference oscillator and includes a stablenonvariable capacitor 208 which sets a stable frequency. This could alsobe a crystal stabilized oscillator if desired.

The second oscillator 202 is tuned by ferroelectric sensing means 210according to the present invention, this particular sensing means beingsimilar to that shown in FIG. 4 of the drawings. Capacitor 210 will ofcourse change capacity under the influence of polar vapors.

The two oscillators are coupled through a mixer 212 so as to produce abeat frequency which is proportional to the capacity change caused bythe amount of polar vapor deposited on thesensingriieans iiilffiisofcotirs e apparent that suitable means is provided for providing accessof the ambient air to the crystal of sensing means 210 for detecting thepresence of polar vapor.

The initial beat frequency of the circuit can be set by tuning theoscillator 200 to the oscillator 202, and this permits a desirable scalefor the humidity range to be set. The output of the sensing means asillustrated in FIG. 7 would be a digital output, since the beatfrequency can drive digital counters and display devices. As a result,the humidity data may be transmitted through a telephone line becauseattenuation or fading of signals does not affect the frequency.

It is apparent from the foregoing that there is provided ac cording tothe present invention new and novel polar vapor sensing means which isresponsive to changes of polar vapor content, such as humidity and'thelike in the ambient environment, and that the output of the circuitsaccording to the present invention may be employed for providing anindication of the vapors or for operating suitable recording equipmentorcontrol devices for operating any suitable desired apparatus.

It is apparent from the foregoing that this invention may be embodied inseveral forms without departing from the spirit or essentialcharacteristics thereof. The present embodiment is illustrative and notrestrictive, and since the scope of the invention is defined bytheappended claims, all changes that fall within the metes and bounds ofthe claims or that form their functional as well as conjointlycooperative equivalents are therefore intended to be embraced by thoseclaims.

What is claimed is:

1. Polar vapor sensing means comprising a body of polarizable metaloxide dielectric material having an active surface layer including meansfor varying the resistivity of the layer in response to contact witirapolar vapor wherebythe resistivity of the surface layer varies between afirst condition in the absence of a polar vapor and a second conditionin the presence of a polar vapor, said varying being at least on theorder of 1210,000, and at least two separate and spaced electricallyconductive electrodes electrically connected with said layer.

2. Apparatus as defined in claim 1 including means for impressing an ACvoltage across said electrodes, and a load impedance connected with saidelectrodes.

3. Apparatus as defined in claim 1, wherein said body is insoluble inwater.

4. Apparatus as defined in claim 2, wherein said body comprises aferroelectric crystal, and the means for impressing said AC voltageprovides a voltage which is smaller but near to the coercive voltage ofthe crystal.

5. Apparatus as defined in claim 1, wherein said body is selected fromthe group consisting of ferroelectric crystal, ferrielectric crystal,ferroelectric glass ceramics, quartz and glassy compositions.

6. Apparatus as defined in claim I, wherein the surface portions of saidbody to which said electrodes are connected are highly polished.

7. Apparatus as defined in claim 1, wherein the thickness of saidsurface layer is within the range of a fraction of a micron up to about20 microns.

8. Apparatus as defined in claim 1, wherein said surface layer providesa continuous surface conductive path between said spaced electrodes.

9. Apparatus as defined in claim 1, wherein said body comprises aferroelectric crystal selected from the group consisting of sodiumpotassium niobate, sodium niobate vanadate, lithium niobate and bariumsodium niobate.

10. Apparatus as defined in claim 1, wherein said body comprises aferroelectric crystal having additive means selected from the groupconsisting of hafnium, zirconium, niobium and tantalum.

11. Apparatus as defined in claim I, wherein said body comprises aferroelectric crystal having additive means therein for enhancingcapacity variation of the crystal, said additive means being selectedfrom the group consisting of sEonTiTIrhT lanthanum, zirconium andchromium.

12. Apparatus as defined in claim 1, wherein said body comprises aferroelectric crystal having additive means therein for enhancingresistivity variation of the crystal, the additive means being selectedfrom the group consisting of lanthanum, niobium, tantalum andmolybdenum.

13. Apparatus as defined in claim 1, wherein said body comprises aferroelectric crystal for sensing polar vapors of crude oils, saidcrystal comprising bismuth titanate having additive means thereinselected from the group consisting of the rare earths.

14. Apparatus as defined in claim 1, wherein the free area of thecrystal is substantially greater than the area of the electrodes, theouter periphery of the electrodes being large relative to the areathereof.

15. Apparatus as defined in claim 1, wherein at least one of saidelectrodes includes a plurality of relatively narrow elongated portionsdefining a large contour for the area thereof.

16. Apparatus as defined in claim 1, wherein said electrodes are eachdisposed on one side of the body, each of the electrodes comprising aplurality of spaced elongated interconnected portions, whereby theelectrodes have a large peripheral dimension for the area thereof.

17. Apparatus as defined in claim 1, wherein said body comprises asingle ferroelectric crystal plate, said electrodes being disposed onopposite faces of said plate, each of said electrodes including aplurality of elongated portions for increas-

1. Polar vapor sensing means comprising a body of polarizable metaloxide dielectric material having an active surface layer including meansfor varying the resistivity of the layer in response to contact with apolar vapor whereby the resistivity of the surface layer vAries betweena first condition in the absence of a polar vapor and a second conditionin the presence of a polar vapor, said varying being at least on theorder of 1: 10,000, and at least two separate and spaced electricallyconductive electrodes electrically connected with said layer. 2.Apparatus as defined in claim 1 including means for impressing an ACvoltage across said electrodes, and a load impedance connected with saidelectrodes.
 3. Apparatus as defined in claim 1, wherein said body isinsoluble in water.
 4. Apparatus as defined in claim 2, wherein saidbody comprises a ferroelectric crystal, and the means for impressingsaid AC voltage provides a voltage which is smaller but near to thecoercive voltage of the crystal.
 5. Apparatus as defined in claim 1,wherein said body is selected from the group consisting of ferroelectriccrystal, ferrielectric crystal, ferroelectric glass ceramics, quartz andglassy compositions.
 6. Apparatus as defined in claim 1, wherein thesurface portions of said body to which said electrodes are connected arehighly polished.
 7. Apparatus as defined in claim 1, wherein thethickness of said surface layer is within the range of a fraction of amicron up to about 20 microns.
 8. Apparatus as defined in claim 1,wherein said surface layer provides a continuous surface conductive pathbetween said spaced electrodes.
 9. Apparatus as defined in claim 1,wherein said body comprises a ferroelectric crystal selected from thegroup consisting of sodium potassium niobate, sodium niobate vanadate,lithium niobate and barium sodium niobate.
 10. Apparatus as defined inclaim 1, wherein said body comprises a ferroelectric crystal havingadditive means selected from the group consisting of hafnium, zirconium,niobium and tantalum.
 11. Apparatus as defined in claim 1, wherein saidbody comprises a ferroelectric crystal having additive means therein forenhancing capacity variation of the crystal, said additive means beingselected from the group consisting of strontium, lanthanum, zirconiumand chromium.
 12. Apparatus as defined in claim 1, wherein said bodycomprises a ferroelectric crystal having additive means therein forenhancing resistivity variation of the crystal, the additive means beingselected from the group consisting of lanthanum, niobium, tantalum andmolybdenum.
 13. Apparatus as defined in claim 1, wherein said bodycomprises a ferroelectric crystal for sensing polar vapors of crudeoils, said crystal comprising bismuth titanate having additive meanstherein selected from the group consisting of the rare earths. 14.Apparatus as defined in claim 1, wherein the free area of the crystal issubstantially greater than the area of the electrodes, the outerperiphery of the electrodes being large relative to the area thereof.15. Apparatus as defined in claim 1, wherein at least one of saidelectrodes includes a plurality of relatively narrow elongated portionsdefining a large contour for the area thereof.
 16. Apparatus as definedin claim 1, wherein said electrodes are each disposed on one side of thebody, each of the electrodes comprising a plurality of spaced elongatedinterconnected portions, whereby the electrodes have a large peripheraldimension for the area thereof.
 17. Apparatus as defined in claim 1,wherein said body comprises a single ferroelectric crystal plate, saidelectrodes being disposed on opposite faces of said plate, each of saidelectrodes including a plurality of elongated portions for increasingthe peripheral dimension relative to the area thereof, and protectivemeans disposed adjacent said crystal and electrodes to prevent damagethereto.
 18. Apparatus as defined in claim 17, wherein said protectivemeans comprises a housing disposed in substantially surroundingrelationship to said crystal and said electrodes, said housing havinghole means formed therethrough for permitting polar vapors to come intointimate contact with said sensing means.